1. Field of the Invention
This invention pertains to the transmission of digital data over conventional electrical power lines and in particular to hardware and software to provide a low bit-error rate, high speed signal in real time digital transmission and reception on a.c. power lines in the presence of repetitive impulse noise.
2. Description of the Prior Art
Carrier-current communication is well know in the prior art. A recent summary of the state of the art, "Intrabuilding Data Transmission Using Power-Line Wiring" appeared in the May 1987 issue of the Hewlett-Packard Journal. It is known that the usable data bandwidth of power lines is higher than what is currently used. The major obstacles are high noise levels, high signal attenuation and impulse noise.
A number of systems have been proposed to enhance data communication over power lines. Japanese laid open application No. 58-77334 discloses an information signal transmitter for the purpose of improving the reliability of information signal transmission by forecasting a period where no noise exists, making use of the periodicity of the noise and transmitting signals during a period in a signal transmission line where no noise of large amplitude is generated. The system detects noise on the power line and through a signal processing circuit generates a pulse signal to represent a period where a large amplitude noise does not exist. The period of a pulse with a constant width representing the tail edge of a process noise signal is taken as the transmission period for an information signal. The system considers both the noise level and the duration of the noise. However, this system uses analog techniques for detection of large amplitude noise and a one-shot timing circuit. The system inherently lacks precision and no mention of data rates is present.
Japanese laid open application No. 57-153844 for a power line carrier wave device proposes an interesting system to avoid the malfunction due to noise by detecting the presence or absence of a signal based on a high frequency noise in a steady state. A high frequency component is superimposed on the power line and detected by the detection circuit. It is then compared with a prestored signal and if the output signal of the detecting circuit is different from the signal stored, an output signal with the presence of a high frequency signal is generated. This signal is used to switch a load on or off. Thus, the load is controlled even if a high frequency noise of the same phase occurs.
The problem to be solved in all such systems is to detect noise on a standard electrical power line, determine if the noise is periodic, and if it is periodic to determine in a predictive manner when it will occur again so that transmission can be blocked during the periods of noise. Hence, the problem is to predict a repeated noise pattern on the power line. For example, in a 60 Hz power line, impulse noise from a light dimmer switch will appear twice every cycle yielding a 120 Hz noise repetition rate. Data transmission cannot occur effectively over a power line during the occurrence of such noise, which is typically 15-30 microseconds or the time of one pulse. However, a 60 Hz pulse lasts for 16,666 microseconds. As a result, the prior systems with imprecise noise detection circuits tend to reduce data transmission rates in the presence of a considerable number of noise pulses in each half cycle or full cycle.
In the case of a.c. lines with a.c. voltages and active light dimmers present on the line, the worst noise impairing signal reception is the impulse noise generated by light switch dimmers. Some existing power line transmission methods do not avoid light dimmer impulses. Due to the received impulse disturbances, data rates must be severely limited, often to 1 kbit/second. One prior art solution proposed is to send pulses whose duration is substantially longer than the noise duration. This results in a relatively slow transmission rate, about 1 kbit/sec. The prior art systems typically avoid jamming noises in the frequency domain by channel hopping to avoid noisy frequency channels.